Vessel examination system

ABSTRACT

A system for on-site, non-destructive examination of the seam welds or other portions of the walls of a vessel, for example, a nuclear reactor pressure vessel, including an instrument carrying, remotely controlled vehicle with magnetic adherence means which may be selectively propelled to any position on the walls of the vessel and including an ultrasonic signal system utilizing a triangulation technique for remotely indicating the position of the vehicle on the vessel.

BACKGROUND

In some instances it is desirable to provide in-service examination oflarge pressure vessels to verify the integrity of the vessel or todiscover any incipient defects so that appropriate repairs can be madebefore failure occurs.

In some cases, for example, where the vessel is relatively inaccessableor hazardous to humans, the use of conventional inspection techniquesand equipment is precluded. A noteable example is a pressure vesselcontainment for a nuclear reactor. Such a pressure vessel may be in theorder of 60 feet in height and 20 feet in diameter with walls of steel 4to 12 inches in thickness. In use, such a vessel is exposed toradioactive fields. Furthermore, such vessels are ordinarily closelysurrounded by heat insulation and a biological shield wall. These andother factors dictate the provision of remotely operable equipment forthe in-service inspection of such vessels.

A method for on-site examination of a nuclear reactor pressure vesselhas been described by L. B. Gross et al in U.S. Pat. No. 3,715,914wherein ultrasonic signal transducers for examining the vessel walls aremounted on a carriage which is movably mounted on tracks at the top andbottom of the sides of the vessel. Use of such an arrangement isdifficult unless the surface to be examined is relatively free ofobstruction, particularly when there is restricted space between thevessel and surrounding structure such as a shield wall. In many cases anumber of pipes of various sizes penetrate the vessel walls, such asfeed lines, recirculation lines and the like. A tracked arrangement isnot well suited for circumventing such obstructions nor for scanning theareas above and below such obstructions.

A tank or vessel scaler device which does not require guide tracks isdescribed by T. E. Cowell et al. in U.S. Pat. No. 3,811,320 whereinindependently driven cross-members are selectively held on the vesselwall by selectively actuatable electromagnets whereby the device can bemoved stepwise across the vessel surface.

Various vehicles which utilize magnetism for adherence of the vehicle tosurfaces have been proposed for a variety of purposes and are shown, forexample, by J. D. Hilliard in U.S. Pat No. 1,262,341 by J. W. Allen inU.S. Pat. No. 2,030,840; by Michito Hiraoku in U.S. Pat. No. 3,777,834.

In general, prior devices are found inadequate or limited in theircapability for providing the desired degree of accuracy, versatility orease of operation in the in-service examination of restricted accessvessels.

It is an object of the present invention to provide a system for theremotely controlled examination of the walls of a vessel including avehicle which may be propelled, around obstacles where necessary, to anyposition on the vessel and wherein the position of the vehicle isdetermined with a high degree of accuracy.

SUMMARY

This and other objects of the invention are achieved by providing asystem including a vehicle fitted with electric motor driven magneticwheels, the wheels on each side being separately driveable by which thevehicle can be steered.

Ultrasonic signal transducers for transmitting signals into andreceiving reflected signals from the vessel material are fastened to acarriage or bracket which, in turn, is translatably supported on a guideframe or track supported by the vehicle by which the transducers may beselectively moved across the surface of the vessel in a directiontransverse to the longitudinal axis of the vehicle whereby a path isscanned across the vessel.

The location of the vehicle on the vessel is accurately determined by anacoustic signal arrangement employing a triangulation techniqueincluding an acoustic signal transmitter mounted on the vehicle and atleast three acoustic signal receiving transducers mounted at knownspaced locations on the vessel.

Other features of the system of the invention include a turningelectromagnet mounted on the vehicle to provide a precise pivot pointwhen turning the vehicle, a pendulum operated indicator for indicatingthe attitude or azimuth of the vehicle with respect to the verticalsurface of the vessel, an independent odometer wheel coupled to anindicator or odometer for indicating distance of vehicle travel and anindependent castor wheel coupled to an indicator for indicating thedegree of offset of the caster wheel with respect to the longitudinalaxis of the vehicle as an indication of turning of the vehicle.

DRAWINGS

Other features and further details of the invention are describedhereinafter with reference to the accompanying drawing wherein:

FIG. 1 is a schematic diagram of the system of the invention;

FIG. 2A is a schematic illustration of the triangulation arrangementused for determination of vehicle location;

FIG. 2B is a timing diagram and FIG. 2C is a sketch of an example of thevehicle location technique;

FIG. 3 is a elevation view, partly in section of a typical pressurevessel surrounded by a shield wall;

FIG. 4 is a perspective view of the vehicle of the system of theinvention; and

FIG. 5 is a perspective view of the vessel inspection transducer blockand carriage.

DESCRIPTION

The system of the invention is broadly illustrated by the schematicdiagram of FIG. 1. The system includes a vehicle 10, a vehicle locationsignal transmitting transducer or transmitter 11 mounted on the vehicle,a plurality of vehicle location signal receiving transducers orreceivers 12(1) - 12(n), a vehicle remote control unit 13, an inspectionsignal processing and recording unit 14 and a computer unit 16.

Vessel inspection transducers are attached to a carriage or bracket 15movably mounted on a transverse guide or track 17 supported by thevehicle 10 and means are provided to selectively move carriage 15 backand forth to scan a path on the vessel to be examined in a directiontransverse to the longitudinal axes of the vehicle 10. The vehicle 10 isfitted with magnetic wheels 18 for adherence of the vehicle to thevertical walls of the vessel (indicated generally as 19) the wheelsbeing driven by selectively energizable electric motors by which thevehicle can be propelled and guided or steered. (Details of the vehiclewill be described hereinafter with reference to FIG. 4.)

A cable of electrical wires 20 connects the location receivertransducers 12(1) - 12(n) to the computer 16. A trailing remote controlcable of electrical wires 21 is connected between the vehicle 10, acable reel and carriage device 22 and the units 13 and 14; and a cableof wires 23 provides electrical and signal interconnection between theunits 13 and 14 and the computer unit 16.

The acoustic signal vehicle location receiving transducers or receivers12(1) - 12(n) are located near the top and bottom of the vertical wallsof the vessel or at other convenient spaced locations. Since theacoustic signal from the transmitting transducer 11 suffers attenuationin its propagation through the vessel material, a sufficient number ofreceivers 12(1) - 12(n) are used so that in any position of the vehicle10 on the portion of the vessel under examination the closest three (orother selected number) of the receivers 12(1) - 12(n) will receivesignals of sufficient strength. (The transducers 12(1) - 12 (n)conveniently may be removably secured to the vessel by, for example,suitable permanent magnets.)

Operation of the vehicle location arrangement is as follows, withreference to the schematic diagram of FIG. 2A and the exampleillustrated by FIGS. 2B and 2C.

Under manual control from the control unit 13 or under automatic controlfrom a computer 31 or computer unit 16, the transmitter 11 periodicallytransmits (for example, by actuation of a pulser 32) a timed pulse orburst 24T of acoustic signal, for example, having a frequency of 350KHz, into the material of the vessel wall (for example, steel) at a timet₀.

This acoustic signal is transmitted through the vessel walls at avelocity depending on the propagation characteristics of the vesselmaterial. This velocity is known or can be determined by routineexperiment.

Thus this acoustic signal pulse will be received by receivingtransducers 12(1) - 12(n) at times dependent upon the distance of theparticular receiving transducers from the vehicle 10 as illustrated inFIG. 2B. As shown in FIG. 1, the receiver 12(5) is closest to thetransmitter 11, therefore, it is first to receive, at a time t₁, thetransmitted location signal as a received signal 24R₁. Receivers 12(4)and 12(2), being more remote, receive the location signal, at times t₂and t₃, as received signals 24R₂ and 24R₃.

The transmitted signal 24T is also received by the other receivers atlater times; however, it is a feature of the present locationarrangement that only the first three (or other predetermined number) ofthe received signals are used for vehicle location determination. Thismay be accomplished, for example, by a counter 33 which counts thereceived signals and disables a gate 34 after the predetermined numberof signals are received, thus gating off subsequent signals during theremainder of the particular vehicle location determination period. Thecounter 33 may be reset by the transmitter actuation pulse from pulser32 at the beginning of the next location determination period.

Thus from the known locations of the receivers 12(1) - 12(n) and thetimes t₁, t₂ and t₃, the location of the vehicle 10 can be determined bya standard graphical technique generally known as triangulation. Forexample, one calculational method of performing such triangulation is asfollows with reference to FIG. 2C: As illustrated in FIG. 2B, t₁ <t₂<t₃. First, calculate a line H₁, a hyperbola all points of which equalt₂ -t₁ for any position of the vehicle along the line H₁. That is, theorigin of the transmitted signal 24T is somewhere along this line. Next,calculate a line H₂, a hyperbola all points of which equal t₃ -t₂. Asillustrated in FIG. 2C, the lines H₁ and H₂ define the location of thetransmitter 11 (and hence the vehicle 10). However, in general, thelines H₁ and H₂ may have two intersections, either of which mayrepresent the vehicle position. Thus, finally, calculate a third lineH₃, a hyperbola all points of which equal t₃ -t₁. This resolves anyambiguity and the location of the vehicle 10 is the common intersectionof lines H₁, H₂ and H₃. By this or other know calculational method thecomputer 31, suitably programmed, can perform the vehicle positioncalculation and provide a vehicle position signal over cable 23 to units13 and 14 for storage and display as desired.

As described above, time t₀ is a reference time based upon thetransmitted acoustic signal pulse 24T. Other reference times can beused. For example, a reference time can be taken as the time that thetransmitted pulse 24T is first received by one of the receivingtransducers 12(1) - 12(n) and the triangulation can then be accomplishedwith reference to this time from the subsequent times that thetransmitted pulse is received at the next three receiving transducers.

Illustrated in FIG. 3 is a typical large pressure vessel 40 formed ofthick steel plates welded together. The vehicle 10 is schematicallyshown on the vertical walls 19 of the vessel. The vessel 40 issurrounded by a biological shield wall 41 formed, for example, ofconcrete which may be spaced from the vessel by as little as 8 or asmuch as 36 inches. The walls 19 of the vessel 40 are penetrated by aplurality of pipes of various sizes such as pipes 42, 43 and 44 whichpresent obstructions that must be circumvented by the vehicle 10.

Receivers 12(1) - 12(n) are shown positioned near the top and bottom ofthe wall 19; however, the receivers can be placed in any convenientlocation on wall 19 spaced around the area in which the vehicle 10 is toscan.

For ease of handling and to minimize the weight of remote control cable21 on the vehicle 10, it is preferably to provide a cable reel andcarriage arrangement 22 conveniently mounted for example, on tracks 46atop the shield wall 41. The cable reel and carriage 22 (not describedin detail) may be any suitable device capable of operating under remotecontrol to pay out or take up the cable 21 as required as the vehicle 10is maneuvered across the vehicle wall 19.

The remotely controlled vehicle 10 is illustrated in FIG. 4. As showntherein a vehicle frame is comprised of a plurality of bars 51-55fastened together in spaced parallel relation by tie rod and spacermembers 56 and 57. (To the extent feasible, the frame members, bracketand carriage members, etc., are preferably formed of light-weightnon-magnetic material such as aluminum.)

The vehicle 10 is supported by four magnetic wheel units 58. Each wheelunit includes a rectangular frame 59, a magnetic wheel 61 having an axlejournalled for rotation in frame 59, and an electric motor 62 coupled todrive the wheel 58.

The wheels 58 contain permanent magnets and are formed, for example,with a plurality of annular pole pieces 63 separated by spacers orwashers 64 of non-magnetic material such as aluminum. The adjacent endsof pole pieces 63 are of opposite poles so that strong magnetic fieldsare developed across the spacer washers 64.

The wheel frames 59 are pivotally attached to the vehicle framework, forexample, the left-rear wheel frame 59 is attached to spaced frame bars51 and 52 by pivot pins such as pivot pin 66. This allows the wheels 58to maintain as flat a position as possible against the cylindricalsurface of the vessel and greatly increases the adherence of the vehicleto the vessel as compared to wheels rigidly mounted to a frame.

Since each wheel 61 is provided with an individual drive motor 62, thevehicle readily may be steered or turned by actuation of the drivemotors on one side or the other or by reversing the motors on one sideas desired.

To provide a definite and known pivot point and to aid in making largedegree turns, a selectively energizeable electromagnet 67 is provided.Electromagnet 67 is attached to a shaft 68 which is journalled forrotation in a bracket 65. The bracket 65 is supported on frame members52 and 53. A pair of springs 72, attached between the bracket 69 and theends of a double arm 70, secured to shaft 68, return the electromagnet67 to its initial position upon de-energization thereof.

The vehicle location transducer or transmitter 11 is mounted in atransducer block 47 (formed on non-magnetic material such as plastic)pivotally supported with a pair of pins 60 by a bracket 69, the bracket69 being pivotally attached to the frame members 53 and 54 by a pair ofpivot pins 71. To assure consistent contact of the contacting face 49 oftransducer block 47 with the vessel wall 19 (and hence good coupling ofthe transmitter 11), the face 49 is fitted at its four corners withpermanent magnets 48.

At its forward end, the vehicle 10 supports a vessel scanningarrangement. Suitable vessel examination transducers are fitted in atransducer block 73 attached to the carriage 15. (Details of the block73 and carriage 15 are described hereinafter with reference to FIG. 5.)

The carriage 15 is mounted on the track or guide arrangement 17 whichincludes one or more guide rods 76 and an elongated drive screw 77, thedrive screw 77 engaging a threaded portion of the carriage 15 by whichrotation of the screw 77 can drive the carriage 15, and hence thetransducer block 73, back and forth, in a direction transverse of thelongitudinal axis of the vehicle 10 whereby the transducers in block 73can scan a path across the surface of the vessel. The guide rods 76 anddrive screw 77 are supported by a pair of end plates 78 which aresecured to frame bar 55. A reversible electric drive motor 79 is gearedto drive screw 77 for selective rotation thereof. A suitable rotationencoder 80 attached to the right-hand end plate 78 is driven by thescrew 77 to indicate rotation of the screw 77 and hence the position ofthe carriage 15.

To provide an indication of the attitude or azimuth of the vehicle onthe verticle walls of the vessel, a pendulum 81 is pivotally attached toa bracket 82 fixed to frame bar 51. The pendulum 81 drives a suitableposition encoder 83, for example, a potentiometer or other suitableposition sensitive device.

The vehicle 10 is also provided with a caster wheel assembly 84 toindicate turning of the vehicle. The caster wheel assembly 84 includes acaster wheel 86 journalled in a caster arm and axle unit 87. The casterwheel 86 contains permanent magnet means, for example, a permanentmagnet tire 88, to provide adherence of the wheel 86 to the vessel. Thecaster axle is journalled in a bracket 89 (fixed to frame bar 51) withsufficient spacing to assure continuous contact of the wheel 86 with thevessel surface irrespective of changes in the distance between the bar51 and the vessel surface.

The caster axle drives a suitable position encoder 91 (for example, apotentiometer) which provides an electrical indication of the positionof the caster wheel with respect to the longitudinal axis of thevehicle. Coupled to the caster axle a selectively actuatable brake 93 isprovided to prevent change in position of the caster wheel when desired,for example, when the vehicle is driven in the reverse direction.

Also provided is a magnetic odometer wheel 85 pivotally attached to thevehicle 10 by arms 90. An odometer or distance travel encoder 92 (ofknown suitable type) is driven by the axle of the odometer wheel 85 toprovide an indication of the distance of vehicle travel. A selectivelyactuable brake 95 is also provided by which rotation of the odometerwheel 85 can be prevented when desired, for example, to maintain thereading of the odometer 92 during vehicle turns.

the vehicle 10 may also carry other desirable instruments, for example,a television camera 94 supported on the vehicle by a bracket 96.

An enclosure 97 is provided as a housing for terminal boards, relays,preamplifiers and the like and as a terminus for the remote controlcable 21. The cable 21 contains the necessary electrical power, controland signal conductors. It also includes a flexible tube for providingcoupling fluid for transmitting transducer 11 and the vessel inspectiontransducers in transducer block 73. For clarity of the drawing, theconnections from cable 21 to the various devices on vehicle 10 are notshown.

The vessel inspection transducer block 73 and its carriage 15 are shownin FIG. 5. The carriage 15 includes a transverse member 101 to which isattached side members 102 to thus form an inverted U-shaped carriage.The member 101 is formed with holes 103 through which the guide rods 76(FIG. 4) extend. Holes 103 may be fitted with suitable bushings (notshown). The member 101 is also formed with a hole 104 which may bethreaded or fitted with a threaded bushing or nut to receive the drivescrew 77 (FIG. 4).

The transducer block 73 includes a body 106 formed of non-metallicmaterial such as a suitable plastic. The body 106 is pivotally attachedto the carriage 15 by an arrangement which also allows limited relativemovement between the carriage 15 and transducer block 73. Thisarrangement includes a pair of pivot pins, such as a pivot pin 107secured to and extending from opposite sides of the body 106. The pins107 are journalled in bores 108 in a pair of connecting blocks 109.

Fixed to the inside surfaces of side members 102 are respective guidebars 111 and the connecting blocks 109 are formed with mating grooves112 by which the guide bars 111 are slideably engaged. With thisarrangement the transducer block 73 is free to follow the contour of acurved vessel wall under inspection.

Fitted in cavities in the body 106 of transducer block 73 is a pluralityof suitable vessel inspection transducers 114(1) - 114(6). Transducers114(1) and 114(2) may be perpendicular to the face 116 of the body 106(and hence substantially perpendicular to the surface of the vessel).Transducers 114(3) and 114(4) may be oriented at different angles fromthe face 116 in the direction of travel of the carriage 15 and block 73;while the transducers 114(5) and 114(6) may be oriented at differentangles transverse to the direction of travel of carriage 74. Forexample, transducers 114(3) and 114(5) may be oriented about 45° (fromthe perpendicular) while the transducers 114(4) and 114(6) may beoriented about 60° (from the perpendicular). Thus, for example, inexamining the weld seams of the vessel, one of the transducers 114(1)and 114(2) can be used to examine the base metal and the other can beused to examine the weld in a perpendicular direction while thetransducers 114(3) - 114(6) can be used to examine the weld seam fromtwo different directions at two different angles.

To assure close contact between the transducer block 73 and the vesselwall, and hence good coupling of the transducers 114(1) - 114(6), thebody 106 is fitted, at the corners of the face 116, with permanentmagnets 113.

Additionally, a tubular fitting 117 attached to the body 106 is providedfor connection to a hose (not shown) by which a suitable acoustic signalcoupling fluid (for example, water) may be fed through suitable channels(not shown) in the body 106 to the faces of the transducers 114(1) -114(6).

The transducers 114(1) - 114(6) are operated in the well-known"pulse-echo" manner, that is, each transducer operates as both atransmitter of pulses of acoustic signal and as a receiver ofreflections back to the transducer of such signals. (As is well known,flaws or discontinuities in the vessel metal are detected by the timeand nature of the reflected signals.)

A method of operation of the system, for example, in the inspection ofthe seam welds of vessels wall 19 is as follows with referenceparticularly to the diagram of the system of FIG. 19. It is contemplatedthat an accurately dimensioned map of wall 19 will have been preparedbefore or during installation of the vessel 40 and further that at leastone index mark, such as an acoustically detectable indentation, has beenplaced on the vessel in a convenient known location. With reference tothe index mark the vehicle 10 is propelled and guided to the weld seamto be examined with the vehicle location transmitter 11 positioned overthe weld seam. The location transmitter 11 is then pulsed and thevehicle location is determined and displayed and recorded and describedhereinbefore.

Starting at a known position (as indicated by encoder 80) the carriage15 and hence the transducer block 73, is moved in discrete steps alongthe carriage guide 17. At each such step, the vessel inspectiontransducers 114(1) - 114(6) are sequentially pulsed with an acousticsignal, of a frequency of 2.25 MHz, for example, and the reflectedsignals from these pulses are received and recorded in known manner.

After the carriage 15 has been thus stepped over a desired distance onguide 17, the drive motors of the vehicle 10 are actuated to drive thevehicle forward (or backward) to thus present another (preferablyoverlapping) scanning path for the vehicle inspection transducers. Thevehicle location transmitter 11 is again pulsed, etc., to start anotheroperating cycle. It is further contemplated that this inspectionprocedure can be carried out both manually and automatically undercomputer control.

Thus what has been described is a system for examining the walls of avessel including a remotely controlled vehicle with position determiningmeans which can be propelled to any selected position on the vesselwall.

What is claimed is:
 1. Apparatus for examining a vessel formed ofmagnetic metal comprising:a remotely controlled vehicle including aframe and having a longitudinal axis; magnetic means for providingadherence of said vehicle to said vessel while allowing movement of saidvehicle over the surface of said vessel; propelling means for propellingsaid vehicle in the direction of said longitudinal axis; steering meansfor turning said vehicle; remotely operable controlling means forcontrolling said propelling and steering means; indicating means forindicating the location of said vehicle on said vessel including;generating means for generating an acoustic vehicle location signal,transmitting means mounted on said vehicle for transmitting saidacoustic signal through the walls of said vessel, at least threeacoustic signal detectors positioned at spaced locations on said vesselfor receiving said acoustic vehicle location signal and for producingoutput signals in response thereto, and triangulation means connected tosaid detectors and responsive to said output signals from said detectorsto provide an indication of the location of said vehicle on said vessel;and scanning means mounted on said frame of said vehicle in proximity tothe surface of said vessel for examining local portions of said vessel.2. Apparatus according to claim 1 wherein said controlling meansincludes means for stepping said vehicle a predetermined distance onsaid vessel.
 3. Apparatus according to claim 1 further including a pivotelectromagnet mounted on said vehicle adjacent to the surface of saidvessel, said electromagnet being selectively energizable to serve as apivot point for and to aid in adherence of said vehicle to said vesselduring turning of said vehicle.
 4. Apparatus according to claim 3wherein said transmitting means for said acoustic vehicle locationsignal is located outboard of said propelling means.
 5. Apparatusaccording to claim 1 wherein said propelling means comprises at leastone wheel mounted at opposite sides of said vehicle, said wheelscontaining said magnetic means in the form of permanent magnets forproviding adherence of said vehicle to said vessel, said propellingmeans including separate electric drive motors for the wheels on saidopposite sides of said vehicle, and selectively actuatable means toenergize said drive motors to provide said steering means.
 6. Apparatusaccording to claim 5 wherein each of said wheels is formed of aplurality of permanent magnets and a plurality of spaced annular polepieces whereby the magnetic force is concentrated at the spaces betweenadjacent ends of said pole pieces.
 7. Apparatus according to claim 6wherein said pole pieces are spaced by washers formed on non-magneticmaterial.
 8. Apparatus according to claim 5 wherein each of said wheelsis rotatably attached to one end of a suspension bracket, the other endof said suspension brackets being pivotally attached to said frame ofsaid vehicle whereby said wheels are free to conform to the surface ofsaid vessel.
 9. Apparatus according to claim 8 wherein the pivot axes ofsaid suspension brackets are substantially parallel to the longitudinalaxis of said vehicle.
 10. Apparatus according to claim 1 wherein saidacoustic vehicle location signal is in the form of pulses, the periodbetween said pulses being greater than the transmission time of one ofsaid pulses through the walls of said vessel between said transmittingmeans and any one of said acoustic signal detectors.
 11. Apparatusaccording to claim 10 further including more than three of said acousticsignal detectors positioned at spaced locations on said vessel, saidtriangulation means including means to provide said indication of thelocation of said vehicle on said vessel in response to the first of apredetermined number of signals received from said detectors during anygiven period.
 12. Apparatus according to claim 11 wherein saidpredetermined number is three.
 13. Apparatus according to claim 1further including a pendulum pivotally attached to said vehicle andmeans operated by said pendulum for indicating the position of saidpendulum with respect to said vehicle.
 14. Apparatus according to claim1 further including a caster wheel including a caster axle for pivotallymounting said caster wheel on said vehicle for rotation over the surfaceof said vessel upon movement of said vehicle over said surface and meansoperated by said caster axle for indicating the position of said casterwheel with respect to said vehicle.
 15. Apparatus according to claim 14wherein said caster wheel is formed with permanent magnet means tomaintain contract of said wheel with said vessel.
 16. Apparatusaccording to claim 15 further including a brake for said caster axle,said brake being selectively operable to prevent change in position ofsaid caster wheel.
 17. Apparatus according to claim 1 further includingan odometer wheel mounted on said vehicle for rotation over the surfaceof said vessel and odometer means connected to said odometer wheel andresponsive to rotation thereof to provide an indication of travel ofsaid vehicle.
 18. Apparatus according to claim 1 wherein said scanningmeans includes at least one acoustic signal transducer coupled to thesurface of said vessel and selectively operable carriage means supportedby said vehicle for selectively moving said transducer across saidsurface of said vessel in a direction transverse of said vehicle. 19.Apparatus according to claim 1 wherein said scanning means includes atleast two acoustic signal transducers coupled to the surface of saidvessel at different angles of orientation with respect to said surfaceand selectively operable carriage means supported by said vehicle forselectively moving said transducers across said surface of said vessel.20. Apparatus according to claim 1 wherein said scanning means includesat least one acoustic signal transducer coupled to the surface of saidvessel, said transducer being mounted in a transducer mounting block,said block having a face adjacent to said surface of said vessel, saidblock being fitted with at least one magnet for slidable adherence ofsaid block to said vessel, said block being attached to selectivelyoperable carriage means supported by said vehicle for selectively movingsaid block, and hence said transducer across said surface of saidvessel.
 21. Apparatus according to claim 20 wherein the attachment ofsaid block to said carriage allows limited movement of said block withrespect to said carriage in a direction substantially normal to saidsurface of said vessel.
 22. Apparatus according to claim 20 wherein saidface of said block is cut out to form a shallow cavity between saidblock and said surface of said vessel and means providing an acousticsignal coupling fluid in said cavity.
 23. Apparatus according to claim20 including a first acoustic signal transducer mounted in said blockwith an orientation substantially normal to said surface of said vessel,a second acoustic signal transducer mounted in said block with anorientation at an angle of less than 90° to said surface of said vehicleand a third acoustic signal transducer mounted in said block with anorientation at a different angle of less than 90° to said surface ofsaid vehicle.
 24. Apparatus according to claim 20 wherein said carriagemeans includes a guide member positioned transverse of and supported bysaid vehicle; a carriage slidably supported on said guide member, saidtransducer mounting block being attached to said carriage; a carriagedrive screw supported by said vehicle and positioned in spacedsubstantially parallel relation to said guide member, a threaded portionof said carriage engagine said drive screw; and a selectively operablereversable drive motor for rotating said drive screw whereby saidtransducer mounting block selectively can be moved back and forth acrossthe surface of said vessel in a direction transverse of the direction ofsaid vehicle.
 25. Apparatus according to claim 24 including a rotationencoder coupled to said drive screw for providing an indication ofrotation of said screw and the position of said carriage on said guidemember.
 26. Apparatus according to claim 1 further including atelevision camera mounted on said vehicle.